Pressure sensitive adhesive comprising (meth) acrylic polymer and amino acid crosslinker

ABSTRACT

A pressure sensitive adhesive composition is described that comprises a (meth)acrylic polymer comprising polymerized units of (meth)acrylic ester monomer having a Tg less than 0° C. and ethylenically unsaturated monomer comprising a pendent functional group crosslinked with an amine or carboxylic acid group of at least one amino acid. Also described are aqueous pressure sensitive adhesive compositions, adhesive-coated articles, and methods.

SUMMARY

In one embodiment, a pressure sensitive adhesive composition isdescribed. The adhesive composition comprises a (meth)acrylic polymercomprising polymerized units of (meth)acrylic ester monomer having a Tgless than 0° C. and ethylenically unsaturated monomer comprising apendent functional group crosslinked with an amine or carboxylic acidgroup of at least one amino acid. In some embodiments, the pressuresensitive adhesive exhibits a 180° peel adhesion to stainless steel ofat least 0.25, 0.5, 1 or 1.5 N/cm. Also described is an articlecomprising the pressure sensitive adhesive disposed on a substrate suchas a release liner or backing.

In another embodiment, a pressure sensitive adhesive composition isdescribed comprising an aqueous phase, a surfactant, a (meth)acrylicpolymer comprising polymerized units of (meth)acrylic ester monomerhaving a Tg less than 0° C. and ethylenically unsaturated monomercomprising a pendent functional group wherein the pendent functionalgroup forms an ionic or covalent bond with an amine or carboxylic acidgroup, and at least one amino acid crosslinker.

In other embodiments, methods of making a pressure sensitive adhesivecomposition are described. In some embodiments, an amino acid iscombined with an aqueous pressure sensitive adhesive composition afterthe (meth)acrylic polymer has been formed. In another embodiment, amonomer premix comprising low Tg (meth)acrylic monomer and monomerhaving a functional group that is reactive with an amine or carboxylicacid group is combined with an amino acid (e.g. comprising a side chainhaving a chain transfer groups such as SH, SeH, or OH) during theformation of the (meth)acrylic polymer. In this later embodied method,the amino acid can function as a chain transfer agent duringpolymerization and as a crosslinker after drying.

DETAILED DESCRIPTION

Herein, “(meth)acrylic” includes both methacrylic and acrylic.

Herein, “(meth)acrylate” includes both methacrylate and acrylate.

Herein, “alkyl” includes straight-chained, branched, and cyclic alkylgroups and includes both unsubstituted and substituted alkyl groups.Unless otherwise indicated, the alkyl groups typically contain from 1 to20 carbon atoms. Examples of “alkyl” as used herein include, but are notlimited to, methyl, ethyl, n-propyl, n-butyl, n-pentyl, isobutyl,t-butyl, isopropyl, n-octyl, 2-octyl, n-heptyl, ethylhexyl, cyclopentyl,cyclohexyl, cycloheptyl, adamantyl, and norbornyl, and the like. Unlessotherwise noted, alkyl groups may be mono- or polyvalent.

When a group is present more than once in a formula described herein,each group is “independently” selected unless specified otherwise.

The pressure sensitive adhesive composition described herein comprises a(meth)acrylic polymer. The (meth)acrylic polymer is prepared fromvarious monomers common to acrylic adhesives, such as a (meth)acrylicacid ester monomers (i.e. a (meth)acrylate ester monomer, also referredto as alkyl (meth)acrylate. The (meth)acrylic copolymer comprisespolymerized units of an ethylenically unsaturated monomer comprising apendent functional group that forms an ionic or covalent bond with anamine or carboxylic acid group of the amino acid crosslinker. Thus, such(meth)acrylic polymer can be characterized as a (meth)acrylic copolymer.The (meth)acrylic copolymer optionally includes various other monomers.

The (meth)acrylic copolymer comprises one or more (meth)acrylate estermonomers derived from an (e.g. non-tertiary) alcohol containing from 1to 22 carbon atoms. Examples of monomers include esters of eitheracrylic acid or methacrylic acid with a non-tertiary alcohol such asethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, 1-pentanol,2-pentanol, 3-pentanol, 2-methyl-1-butanol, 3-methyl-1-butanol,1-hexanol, 2-hexanol, 2-methyl-1-pentanol, 3-methyl-1-pentanol,2-ethyl-1-butanol, 3,5,5-trimethyl-1-hexanol, 3-heptanol, 1-octanol,2-octanol, isooctyl alcohol, 2-ethyl-1-hexanol, 1-decanol,2-propylheptanol, 1-dodecanol, 1-tridecanol, 1-tetradecanol,citronellol, dihydrocitronellol and the like.

The pressure sensitive adhesive comprises one or more (meth)acrylic acidester monomers having a low glass transition temperature (Tg) less than0° C. when reacted to form a homopolymer. Suitable low Tg monomerstypically have a Tg less than −10° C., −20° C., −30° C., or −40° C. whensuch is monomer reacted to form a homopolymer. The Tg of thesehomopolymers is often greater than or equal to −80° C., greater than orequal to −70° C., or greater than or equal to −60° C., or greater thanor equal to −50° C.

The low Tg alkyl acrylate monomer may have the following formula

H₂C═CR¹C(O)OR²

wherein R¹ is hydrogen or methyl and R² is alkyl or heteroalkyl with 1to 22 carbons. The alkyl or heteroalkyl group can be linear, branched,cyclic, or a combination thereof.

Exemplary low Tg alkyl acrylates include for example ethyl acrylate,n-propyl acrylate, n-butyl acrylate, isobutyl acrylate, t-butylacrylate, n-pentyl acrylate, isoamyl acrylate, n-hexyl acrylate,2-methylbutyl acrylate, 4-methyl-2-pentyl acrylate, n-octyl acrylate,isooctyl acrylate, 2-ethylhexyl acrylate, isononyl acrylate, decylacrylate, isodecyl acrylate, lauryl acrylate, isotridecyl acrylate,octadecyl acrylate, and dodecyl acrylate.

Exemplary low Tg heteroalkyl acrylates include, but are not limited to,2-methoxyethyl acrylate and 2-ethoxyethyl acrylate.

In some embodiments, the low Tg alkyl methacrylates include an alkylgroup with greater than 4, 5, 6, 7 or 8 carbon atoms. Exemplary alkyl(meth)acrylates include, but are not limited to, 2-ethylhexyl(meth)acrylate, isooctyl (meth)acrylate, n-octyl (meth)acrylate,isodecyl (meth)acrylate, and lauryl (meth)acrylate.

In some embodiments, the monomer is an ester of (meth)acrylic acid withan alcohol derived from a renewable source. A suitable technique fordetermining whether a material is derived from a renewable resource isthrough ¹⁴C analysis according to ASTM D6866-10, as described inUS2012/0288692. The application of ASTM D6866-10 to derive a “bio-basedcontent” is built on the same concepts as radiocarbon dating, butwithout use of the age equations. The analysis is performed by derivinga ratio of the amount of organic radiocarbon (¹⁴C) in an unknown sampleto that of a modern reference standard. The ratio is reported as apercentage with the units “pMC” (percent modern carbon).

One suitable monomer derived from a renewable source is 2-octyl(meth)acrylate, as can be prepared by conventional techniques from2-octanol and (meth)acryloyl derivatives such as esters, acids and acylhalides. The 2-octanol may be prepared by treatment of ricinoleic acid,derived from castor oil, (or ester or acyl halide thereof) with sodiumhydroxide, followed by distillation from the co-product sebacic acid.Other (meth)acrylate ester monomers that can be renewable are thosederived from ethanol and 2-methyl butanol.

In some embodiments, the (meth)acrylic copolymer may further comprise ahigh Tg alkyl (meth)acrylate monomer, having a Tg of at least 0° C., 25°C., or 50° C. Suitable high Tg monomers include, for example, t-butylacrylate, methyl methacrylate, ethyl methacrylate, isopropylmethacrylate, n-butyl methacrylate, isobutyl methacrylate, s-butylmethacrylate, t-butyl methacrylate, stearyl methacrylate, phenylmethacrylate, cyclohexyl methacrylate, isobornyl acrylate, isobornylmethacrylate, benzyl methacrylate, 3,3,5 trimethylcyclohexyl acrylate,cyclohexyl acrylate, and propyl methacrylate or combinations.

The Tg of the copolymer may be estimated by use of the Fox equation,based on the Tgs of the homopolymer of constituent monomers and theweight percent thereof.

In some embodiments, the (meth)acrylic copolymer comprises at least 50,55, 60, 65, 70, 75, 80, 85, 90 or 95 wt.-% (meth)acrylate ester monomershaving a Tg less than 0° C., −10° C., −20° C., −30° C. or −40° C. basedon the total weight of the (meth)acrylic copolymer. When high Tgmonomers are included, the (meth)acrylic polymer may include at least 1,2, 3, 4, 5, 10, 15, 20, ranging up to 30 wt.-% of such high Tgmonomer(s).

Due to the relatively high concentration of polymerized units of low Tgmonomer(s), the (meth)acrylic polymer and pressure sensitive adhesivecomposition also has a Tg less than 0° C. and more typically less than−10° C., −20° C., −30° C. or −40° C. The Tg of the (meth)acrylic polymeris typically at least about −80° C., −70° C., −60° C., or −50° C.

The (meth)acrylic copolymer comprises polymerized units of one or moreethylenically unsaturated monomers comprising a pendent functional groupthat forms an ionic or covalent bond with an amine or carboxylic acidgroup of the amino acid crosslinker. In some embodiments, the pendentfunctional group is a carboxylic acid, amine, or amide. In thisembodiment, such ethylenically unsaturated monomers are a subset ofpolar monomers.

In some embodiments, the (meth)acrylic copolymer comprises polymerizedunits derived from an acid-functional ethylenically unsaturated monomer,where the acid-functional group may be an acid per se, such as acarboxylic acid, or a portion may be a salt thereof, such as an alkalimetal carboxylate. Useful acid-functional ethylenically unsaturatedmonomers include, but are not limited to, those selected from anethylenically unsaturated carboxylic acids, ethylenically unsaturatedsulfonic acids, ethylenically unsaturated phosphonic acids, and mixturesthereof. Such acid groups can react with the amine group of the aminoacid crosslinker. Examples of such compounds include those selected fromacrylic acid (AA), methacrylic acid, itaconic acid, fumaric acid,crotonic acid, citraconic acid, maleic acid, β-carboxyethyl(meth)acrylate, 2-sulfoethyl methacrylate, styrene sulfonic acid,2-acrylamido-2-methylpropanesulfonic acid, vinylphosphonic acid, andmixtures thereof.

Acid-functional ethylenically unsaturated monomers are typicallyselected from ethylenically unsaturated carboxylic acids, i.e.,(meth)acrylic acids. When an even stronger acid is desired, anacid-functional ethylenically unsaturated monomer includes anethylenically unsaturated sulfonic acid, an ethylenically unsaturatedphosphonic acid, or a mixture thereof.

In some embodiments, the (meth)acrylic copolymer comprises polymerizedunits of acid-functional ethylenically unsaturated monomer in an amountof at least 0.5, 1, 2 or 3 wt.-%, based on the total weight of the(meth)acrylic copolymer. The polymerized units of acid-functionalethylenically unsaturated monomer can be present in an amount up toabout 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, or 15 wt.-%. However, when the(meth)acrylic copolymer comprises polymerized units comprising pendentamine or amide groups, the (meth)acrylic copolymer may comprise lessthan 0.5 wt.-%, less than 0.1 wt.-% or zero polymerized units ofacid-functional ethylenically unsaturated monomer.

One exemplary (meth)acrylic copolymer comprising carboxylic acidfunctionality is derived from copolymerizing isooctyl acrylate (IOA) andacrylic acid (AA).

The adhesive copolymer may further comprise polymerized units of one ormore polar non-acid-functional monomers. The polar monomers useful inpreparing the copolymer are both somewhat oil soluble and water soluble,resulting in a distribution of the polar monomer between the aqueous andoil phases in an emulsion polymerization.

Representative examples of suitable non-acid functional polar monomersinclude but are not limited to 2-hydroxyethyl (meth)acrylate;N-vinylpyrrolidone; N-vinylcaprolactam; acrylamide; mono- or di-N-alkylsubstituted acrylamide; t-butyl acrylamide; dimethylaminoethylacrylamide; N-octyl acrylamide; poly(alkoxyalkyl) (meth)acrylatesincluding 2-(2-ethoxyethoxy)ethyl (meth)acrylate, 2-ethoxyethyl(meth)acrylate, 2-methoxyethoxyethyl (meth)acrylate, 2-methoxyethylmethacrylate, polyethylene glycol mono(meth)acrylates; alkyl vinylethers, including vinyl methyl ether; and mixtures thereof.

Some of such non-acid functional polar monomers provide pendent amine oramide groups to the (meth)acrylic polymer. Such amine or amide groupsreact with the carboxylic acid group of the amino acid crosslinker.Representative monomers include acrylamide; mono- or di-N-alkylsubstituted acrylamide; t-butyl acrylamide; dimethylaminoethylacrylamide; and N-octyl acrylamide.

The non-acid functional polar monomer(s) that provide pendent amine oramide groups may be present in an amount of at least 0.5, 1, 2 or 3wt.-%, based on the total weight of the (meth)acrylic copolymer. Suchpolymerized units of polar monomer can be present in an amount up toabout 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, or 15 wt.-%. However, when the(meth)acrylic copolymer comprises polymerized units comprisingpolymerized units of acid-functional ethylenically unsaturated monomer,the (meth)acrylic copolymer may comprise less than 0.5-wt.%, less than0.1 wt.-% or zero of polar monomer(s) that provide pendent amine oramide groups. The (meth)acrylic polymer may optionally comprise othernon-acid functional polar monomer(s) that lack pendent amine or amidegroups.

The (meth)acrylic copolymer may optionally comprise polymerized units ofone or more vinyl monomers such as vinyl esters (e.g., vinyl acetate andvinyl propionate), styrene, substituted styrene (e.g., .alpha.-methylstyrene), vinyl halide, and mixtures thereof. The (meth)acryliccopolymer may comprise at least 1, 2, 3, 4, or 5 wt.-% up to 10 or 15wt.-% of polymerized units of vinyl monomers. In some embodiments, the(meth)acrylic copolymer comprises less than 0.5-wt.%, less than 0.1wt.-% or zero of vinyl monomer(s).

The aqueous pressure sensitive adhesive composition further comprises atleast one amino acid crosslinker. The amino acid is preferably misciblewith an aqueous phase (meth)acrylic polymer such that the amino aciddisperses without gelling or forming aggregates.

Various amino acids are known. Amino acids comprise a carboxylic acidgroup, an amine group, and a side chain. Amino acids most commonly havethe general structure:

HOC(O)C(NH₂)—R

wherein R is an organic side chain.

In some embodiments, the amino acid crosslinker comprises a hydrophobicside chain such as in the case of alanine, isoleucine, leucine,methionine, phenylalanine, tryptophan, tyrosine, and valine.

In other embodiments, the amino acid crosslinker comprises a cyclicgroup such as in the case or proline, tyrosine, tryptophan, andespecially histidine and phenylalanine and depicted as follows:

In some embodiments, the cyclic group is aromatic or in other words theside chain of the amino acid comprises an aromatic group (e.g. phenyl).In other embodiments, the side chain of the amino acid comprises ahetercyclic group, typically including one or two nitrogen atoms such asin the case of histidine.

In another embodiment, the amino acid comprises an electrically chargedside chain (i.e. electrically charged at a pH of 7.4). For example,lysine, histadine and arginine have a side chain that comprise apositively charge (nitrogen-containing) group. Aspartic acid andglutamic acid have a side chain that comprises a negatively charged(oxygen-containing) group.

In yet other embodiments, the side chain of the amino acid comprises agroup that is suitable to function as a chain transfer agent during thepolymerization of (meth)acrylic polymer. In this embodiment, the aminoacid comprises a chain transfer group such as SH, SeH, or OH such as inthe case of serine, threonine, tyrosine, and cysteine depicted asfollows:

Combinations of two or more amino acid crosslinkers can be utilized.

The concentration of amino acid crosslinker can vary. In typicalembodiments, the concentration of amino acid crosslinker is at least0.005, 0.01, or 0.015, or 0.02 wt.-% solids of the (meth)acrylic polymeror total adhesive composition (such as when tackifier is present). Insome embodiments, the concentration of amino acid crosslinker is atleast 0.1, 0.2, 0.3, 0.4, or 0.5 wt.-% solids of the (meth)acrylicpolymer or total adhesive composition. In other embodiments, theconcentration of amino acid crosslinker is at least 0.6, 0.7, 0.8, 0.9,or 1.5 wt.-% solids of the (meth)acrylic polymer or total adhesivecomposition. In typical embodiment, the concentration of amino acidcrosslinker is no greater than 10, 9, 8, 7, 6, 5, 4, or 3 wt.-% solidsof the (meth)acrylic polymer or total adhesive composition.

The pressure sensitive adhesive composition may optionally compriseadditional crosslinkers such as multifunctional (meth)acrylates,aziridine crosslinkers, and (e.g. chlorinated triazine crosslinkers.However, in typical embodiments, the amino acid is the sole or primarycrosslinker. Thus, the pressure sensitive adhesive composition cancomprise little or no of such additional crosslinkers.

The (meth) acrylic polymer of the pressure sensitive adhesive can beprepared by conventional free radical polymerization method, includingwater-based dispersion, emulsion, and suspension processes. The(meth)acrylate polymers may be prepared via suspension polymerizationsas disclosed in U.S. Pat. Nos. 3,691,140 (Silver); 4,166,152 (Baker etal.); 4,636,432 (Shibano et al); 4,656,218 (Kinoshita); and 5,045,569(Delgado).

Water-soluble and oil-soluble thermal initiators useful in preparing the(meth)acrylate adhesive polymers used in the present invention areinitiators that, on exposure to heat, generate free-radicals whichinitiate (co)polymerization of the monomer mixture. Water-solubleinitiators are preferred for preparing the (meth)acrylate polymers byemulsion polymerization.

Suitable water-soluble initiators include but are not limited to thoseselected from the group consisting of potassium persulfate, ammoniumpersulfate, sodium persulfate, and mixtures thereof; oxidation-reductioninitiators such as the reaction product of the above-mentionedpersulfates and reducing agents such as those selected from the groupconsisting of sodium metabisulfite and sodium bisulfite; and4,4′-azobis(4-cyanopentanoic acid) and its soluble salts (e.g., sodium,potassium). The preferred water-soluble initiator is potassiumpersulfate. Suitable oil-soluble initiators include but are not limitedto those selected from the group consisting of azo compounds such asVAZO™ 64 (2,2′-azobis(isobutyronitrile)), VAZO™ 67 (2,2′azobis(2-methylbutyronitrile)), and VAZO™ 52(2,2′-azobis(2,4-dimethylpentanenitrile)), available from E.I. du Pontde Nemours Co., peroxides such as benzoyl peroxide and lauroyl peroxide,and mixtures thereof. The preferred oil-soluble thermal initiator is2,2′-azobis(2,4-dimethylpentanenitrile). When used, initiators aretypically present in amount of at least about 0.05, 0.1, or 0.5 up toabout 1 part by weight based on the total monomers of the (meth)acrylicpolymer.

The copolymerizable emulsion mixture may optionally further comprisechain transfer agents to control the molecular weight of the resultantpolymer. Examples of useful chain transfer agents include but are notlimited to those selected from the group consisting of carbontetrabromide, alcohols, mercaptans, and mixtures thereof. When present,the preferred chain transfer agents are isooctylthioglycolate and carbontetrabromide. The emulsion mixture may further comprise a chain transferagent in an amount of at least 0.01, 0.02, 0.03, 0.04. or 0.05 up to0.2, 0.3, 0.4, or 0.5 parts by weight based on the total monomer of the(meth)acrylic polymer

Preferably, an emulsion polymerization method is used to prepare theadhesive copolymer. Polymerization via emulsion techniques may requirethe presence of an emulsifier (which may also be called an emulsifyingagent or a surfactant). Useful emulsifiers for the present inventioninclude those selected from the group consisting of anionic surfactants,cationic surfactants, nonionic surfactants, zwitterionic surfactants,and mixtures of surfactants.

The emulsion polymerization is typically carried out in the presence ofanionic surfactant(s). A useful range of emulsifier concentration isfrom about 0.5 to about 8 weight percent, preferably from about 1 toabout 5 weight percent, based on the total weight of all monomers of theemulsion pressure sensitive adhesive.

In emulsion polymerization a reaction occurs in micelles or emulsionmicrodrops suspended in aqueous medium. Any heat generated in themicrodrops or micelles is quickly moderated by the effect of the heatcapacity of the surrounding water phase. Emulsion polymerizationproceeds with better control of exothermic reactions, and the resultingadhesive composition is non-flammable as the aqueous medium is thedominant component.

The (meth)acrylate copolymers may be prepared by a batch, continuous orsemi-continuous emulsion polymerization process. The polymerizationgenerally comprises the steps of:

(a) making a monomer mixture comprising

(i) a sufficient amount of low Tg (meth)acrylic ester monomer(s),

(ii) monomer(s) comprising a functional group that is reactive with acarboxylic acid or amine group;

(iii) optional monomers,

(iv) optional (e.g. oil-soluble) chain transfer agent,

(b) combining the monomer mixture with an aqueous phase comprising

(i) water,

(ii) surfactant(s),

(iii) free radical initiator(s),

(iv) optional (e.g. water soluble) chain transfer agent,

(c) concurrently agitating and heating said emulsion to a temperature ofabout 30° C. to about 80° C. to polymerize the monomers in theoil-in-water emulsion until a polymeric latex is formed. It will beunderstood that once the emulsion mixture is prepared, the monomers maypartition between the oil phase and the water phase, according to theirrespective partition coefficients.

In an alternative embodiment, the monomer(s) comprising the functionalgroup that is reactive with a carboxylic acid or amine group may bepresent in the aqueous phase, rather than the monomer mixture.

In the semicontinuous process, a flask is charged with a seed monomermixture comprising deionized (DI) water, surfactant, acid functionalmonomers, (meth)acrylate ester monomers, optional co-polymerizablemonomers, including optional polar monomers, vinyl monomer, and anyoptional chain transfer agents, pH modifiers or other additives. Themixture is stirred and heated under an inert atmosphere such as anitrogen blanket. When the mixture has reached induction temperature,typically about 50° to about 70° C., the first initiator is added toinitiate the polymerization and the reaction is allowed to exotherm.After the seed reaction is completed, the batch temperature is thenraised to the feed reaction temperature, about 70° to about 85° C. Atthe feed reaction temperature, the monomer pre-emulsion comprisingdeionized water, surfactant acid functional monomers, (meth)acrylateester monomers, optional co-polymerizable monomers, including optionalpolar monomers, chain transfer agents or other additives is added to thestirred flask over a period of time, typically 2 to 4 hours, while thetemperature is maintained. At end of the feed reaction, the secondinitiator charge, if used, is added to the reaction to further reduceresidual monomers in the emulsion. After an additional hour of heating,the mixture is cooled to room temperature (about 23° C.) and theemulsion is collected for evaluation.

A neutralizing agent may be employed in the preparation of thiscopolymer. It may be employed at a level sufficient to neutralize all ora part of the acid groups of the polymer. Neutralization is achieved viathe use of an alkali metal hydroxide or a combination of an alkali metalhydroxide with a minor amount of another neutralizing agent. A widevariety of other neutralizing agents may be used as will be understoodby those skilled in the art. The selection of the other neutralizingagent, and the amount employed may be varied to achieve a desiredresult. However, the type and amount is selected such that the(meth)acrylic polymer remains water dispersible. Preferably ammonium,sodium and potassium hydroxide are used as neutralizing agents.

The pH of the emulsion may be 4 or greater. The acidity of the emulsionmay be modified following latex formation using a pH modifier such as abasic solution (e.g., solutions of sodium hydroxide, ammonium hydroxide,lithium hydroxide and the like) or buffer solutions (e.g., sodiumbicarbonate and the like), to the desired pH levels.

In typical embodiments, the amino acid is added after the (meth)acrylicpolymer is formed and prior to removing the aqueous phase. However, whenthe amino acid comprises a side chain having a chain transfer group theamino acid can be added to the monomer mixture or aqueous phase duringthe polymerization of the (meth)acrylic copolymer. In this embodiment,the amino acid can function as a chain transfer agent during thepolymerization of the monomers and also function as a crosslinker forthe (meth)acrylic polymer. When the amino acid is utilized as a chaintransfer agent during polymerization, the concentration of amino acid isless than 0.1 wt.-% and in some embodiments no greater than 0.075 or0.005 wt.-%. However, additional amino acid can be added afterpolymerization if desired.

The pressure sensitive adhesive may contain one or more conventionaladditives. Preferred additives include tackifiers, plasticizers, dyes,antioxidants, and UV stabilizers.

Suitable tackifiers for use with (meth)acrylate polymer dispersionsinclude rosin acids, rosin esters, terpene phenolic resins, hydrocarbonresins, and coumarone indene resins. The type and amount of tackifiercan affect properties such as contactability, bonding range, bondstrength, heat resistance and specific adhesion. The tackifier willgenerally be used in the form of an aqueous dispersion. Commerciallyavailable tackifiers that are suitable include TACOLYN™ 1070, 5001 and5002 (aqueous, 55% solids synthetic resin dispersions based on lowmolecular weight thermoplastic resins, available from Hercules Inc.),SE1055™ (an aqueous dispersion of a rosin ester, available from HerculesInc.), ESCOREZ™ 9271 (an aliphatic hydrocarbon resin emulsion, availablefrom Exxon), DERMULSENE™ 82, DERMULSENE™ 92, DERMULSENE™ DT orDERMULSENE™ DT50 (aqueous dispersions of modified terpene phenolicresins, available from DRT) and AQUATAK™ 4188 (a modified rosin ester,available from Arizona Chemical Company). When present, the amount oftackifying resin is typically at least 1, 2, 3, 4, or 5 wt-% of thepressure sensitive adhesive and no greater than about 50 or 40 wt-% ofthe pressure sensitive adhesive. In some embodiments, the amount oftackifying resin is at least 10, 15 or 20 wt-% of the pressure sensitiveadhesive. In other embodiments, the amount of tackifying resin is lessthan 1 wt.-% or 0.5 wt.-% or zero.

The (meth)acrylic copolymer typically has a storage modulus (G′) at 25°C. and a frequency of 1 Hertz (radian/second) of at least 1 X 10⁴dynes/cm² and no greater than 1×10⁷ dynes/cm² (1 MPa) or 5×10⁶ dynes/cm²(0.5 MPa) or 3×10⁶ dynes/cm² (0.3 MPa) as determined by DynamicMechanical Analysis. For example, the storage modulus of a (meth)acryliccopolymer prepared from 90 parts isooctyl acrylate, 10 parts acrylicacid is about 1.5×10⁶ dynes/cm² (prior to crosslinking with the aminoacid). In yet another example, a (meth)acrylic copolymer prepared from98 parts isooctyl acrylate and 2 parts acrylic acid is about 2×10⁵dynes/cm² (prior to crosslinking with the amino acid). When the(meth)acrylic polymer alone is not a PSA, the composition can furthercomprise a component such as tackifying resin to reduce the storagemodulus. Although the crosslinking raises the storage modulus, thecrosslinked pressure sensitive adhesive falls within the storage modulusrange of a PSA (e.g. no greater than 3×10⁶ dynes/cm² (0.3 MPa).

In some embodiments, the (dried and crosslinked) pressure sensitiveadhesive exhibits a 180° peel adhesion to stainless steel of at least0.25, 0.5, 1 or 1.5 N/cm. In some embodiments, the 180° peel adhesion tostainless steel does not exceed 5 N/cm. The shear adhesion strength canvary depending on the (meth)acrylic polymer and concentration ofcrosslinker. In some embodiments, the shear adhesion strength tostainless steel and or fiberboard exceeds 10,000 minutes. In someembodiments, the pressure sensitive adhesive exhibits high tack at coldtemperatures (e.g. 40° F.). In some embodiment, the pressure sensitiveadhesive exhibits a shear strength after aging for 10 days at 120° F. ofgreater than or equal to the initial shear strength. In someembodiments, the (initial or aged at 120° F. for 10 or 11 days) shearstrength to fiberboard is at least 25, 50, 100, 150, 200, 250, 300, 350,400, 450, 500, 550, 600, 650, or 700 minutes. Such adhesive propertiescan be determined according to the test methods described in theforthcoming examples

The aqueous emulsions are of a suitable concentration for subsequentcoating. In typical embodiments, the aqueous pressure sensitive adhesivecomposition comprises 30 to 70 wt.-% total solids. Thus the aqueouspressure sensitive adhesive composition contains 30 to 70 wt-% of anaqueous phase. The desired concentration may be achieved by furtherdilution of the coating composition, or by partial drying.

The aqueous phase may contain a small amount of volatile,non-polymerizable organic solvent that may be included in the aqueousadhesive composition to dissolve components, such as the amino acidcrosslinker. The aqueous adhesive composition typically contains lessthan 10, 5, 4, 3, 2, or 1 weight percent of organic solvent.

The above-described aqueous pressure sensitive adhesive compositions arecoated on a substrate, such as a release liner or (e.g. flexible orinflexible) backing material by conventional coating techniques. Thebacking material may be surface treated or comprise a primer layer asknown in the art (See for example WO2014/127341).

The compositions can be applied to a variety of substrates by methodssuch as roller coating, flow coating, dip coating, spin coating, spraycoating knife coating, and die coating. Upon drying, the amino acidcrosslinks with the functional groups of the (meth)acrylic polymer toproduce adhesive coated articles such as sheet materials.

The thickness of the (dried) PSA layer is typically at least about 10,15, 20, or 25 micrometers and no greater than 1500 micrometers (60 mil),1000 micrometers (40 mils), or 500 micrometers (20 mils). Multiplecoatings can be utilized to increase the thickness as desired.

The flexible backing material may be any material conventionallyutilized as a tape backing, optical film or any other flexible material.

Examples of materials that can be included in the flexible supportinclude polyolefins such as polyethylene, polypropylene (includingisotactic polypropylene), polystyrene, polyester, polyvinyl alcohol,poly(ethylene terephthalate), poly(butylene terephthalate), polyimide,poly(caprolactam), poly(vinylidene fluoride), polylactides, celluloseacetate, and ethyl cellulose and the like.

Commercially available backing materials include for example HOSTAPHAN3SAB, primed polyester film (available from Mitsubishi Polyester FilmInc., Greer, S.C.), Kraft paper (available from Monadnock Paper, Inc.);cellophane (available from Flexel Corp.); spun-bond poly(ethylene) andpoly(propylene), such as TYVEK and TYPAR (available from DuPont, Inc.);and porous films obtained from poly(ethylene) and poly(propylene), suchas TESLIN (available from PPG Industries, Inc.), and CELLGUARD(available from Hoechst-Celanese).

Backings may also be prepared of fabric such as woven fabric formed ofthreads of synthetic or natural materials such as cotton, nylon, rayon,glass, ceramic materials, and the like or nonwoven fabric such as airlaid webs of natural or synthetic fibers or blends of these. The backingmay also be formed of metal, metallized polymer films, or ceramic sheetmaterials and may take the form of any article conventionally known tobe utilized with pressure sensitive adhesive compositions such aslabels, tapes, signs, covers, marking indicia, and the like.

The flexible support may also comprise a release-coated substrate. Suchsubstrates are typically employed when an adhesive transfer tape isprovided. Examples of release-coated substrates are well known in theart and include, by way of example, silicone-coated kraft paper and thelike. Tapes of the invention may also incorporate a low adhesionbacksize (LAB) which are known in the art.

Objects and advantages of this disclosure are further illustrated by thefollowing examples, but the particular materials and amounts thereofrecited in these examples, as well as other conditions and details,should not be construed to unduly limit this disclosure.

Materials

Material Description IOA Isooctyl acrylate. AA Acrylic Acid, 99% purity,obtained from Alfa Aesar, Ward Hill, MA. DS-10 Sodium dodecylbenzenesulfonate, available under the trade designation RHODACAL DS-10, fromRhodia Incorporated, Cranbury, NJ. CBr4 Carbon tetrabromide, 98% purity,obtained from Alfa Aesar, Ward Hill, MA. L-Cys L-Cysteine, meltingpoint: 224-226° C., obtained from MP Biomedicals Incorporated, Eschwege,Germany. FS solution 0.22 grams ferrous sulfate, Fe2SO4•7H2O, 99% purityin 100 grams deionized water, from J. T. Baker Chemicals, Phillipsburg,NJ,. SMBS Sodium metabisulfite, Na2S2O5 97% purity, obtained from AlfaAesar, Ward Hill, MA. KPS Potassium persulfate, K2S2O8, obtained fromAlfa Aesar, Ward Hill, MA. LiOH Lithium Hydroxide, 98% purity, obtainedfrom Alfa Aesar, Ward Hill, MA. L-Phenyl L-Phenylalanine, 99% purity,obtained from Alfa Aesar Avocado Organics, Heysham, Lancashire, UK.L-Hist L-Histidine, USP grade, obtained from aMReSCO, Solon, OH. BaseEmulsion A milky white, water-based acrylic emulsion pressure sensitiveadhesive, Polymer 1 53-57% solids, available under trade designationFASTBOND Insulation Adhesive 49 from 3M Company, St. Paul, MN. BaseEmulsion A milky white, anionic emulsion, 53% solids, of a permanentacrylic Polymer 2 pressure sensitive adhesive, available under the tradedesignation ROBOND PS-90 from Dow Chemical Company, Midland, MI. PrimedA 25 micrometer (0.001 inch) thick corona treated polyester film wasPolyester Film provided with a primer coating as described in thepublication JP H02200476A.

Tests Methods Peel Adhesion

Peel adhesion was measured at an angle of 180 degrees as follows. Sampleconditioning and testing were conducted at 23° C. (73° F.) and 50%relative humidity (RH). After conditioning for 24 hours a tape samplemeasuring 12.7-millimeters (0.50 inches) wide and at least 10.2centimeters (4 inches) long was cut. A stainless steel plate wasattached around its borders to the platen of a peel tester using singlecoated PSA tape and wiped twice with isopropanol. Next, the tape samplewas applied to the stainless steel plate and rolled down twice in eachdirection using a 2 kilogram (4.4 pound) rubber roller. The sample waspeeled from the stainless steel plate at a platen speed of 305millimeters/minute (12 inches/minute) over a length of 2.54 centimeters(1 inch) using an IMASS Slip/Peel Tester (Model SP-2000, available fromIMASS Incorporated, Accord, Mass.). Peel adhesion data was collectedover the last 2.0 cm (0.8 inches). Four samples were evaluated, theresults normalized to ounces/inch (oz/in.), and the average reported.

Shear Strength at Room Temperature—Stainless Steel

Shear strength on stainless steel was measured as follows. Conditioningand testing were done at 23° C. (73° F.) and 50% relative humidity (RH).After conditioning for 24 hours a tape sample measuring 12.7 millimeters(0.50 inches) wide and 11.4 centimeters (4.5 inches) long was cut. Astainless steel plate was wiped twice with isopropanol. The tape samplewas then centered on cleaned stainless steel panel and adhered to oneend such that tape overlapped the panel by 12.2 millimeters (0.5 inches)in the lengthwise direction and rolled down twice in each directionusing a 2 kilogram (4.4 pound) rubber roller. A 1.0 kilogram (2.2 pound)weight was then attached to the free end of the tape, and thepanel/tape/weight assembly was suspended in a stand at an angle of 2°from vertical. The time, in minutes, for the tape to fall from the panelwas recorded. The test was terminated if failure had not occurred in10,000 minutes and the result recorded as “10000”. The average of threesamples was reported.

Shear Strength at Room Temperature—Fiberboard

Shear strength of a tape sample on a 127 micrometer (0.005 inches) thickfiberboard was measured in the same manner as described in “ShearStrength at Room Temperature—Stainless Steel” with the followingmodification. A piece of fiberboard measuring 2.54 cm (1 inch) squarewas attached to the stainless steel plate using double coated adhesivetape. The tape sample was then adhered to the exposed fiberboardsurface.

Preparation of Comparative Composition Example 1 (CE 1)

Comparative Composition Example 1, comprising an emulsion polymer EP 1containing CBr4 chain transfer agent (CTA), was made using conventionalemulsion polymerization techniques. The monomer formulation wasIOA:AA/95:5 (w:w). An aqueous solution was prepared by adding 3.8 gramsDS-10 surfactant to 224.7 grams of deionized water in a 1 liter reactionflask equipped with stirrer, reflux condenser, thermometer, infraredheat lamps connected to a temperature controller, and nitrogen purge.The mixture was stirred at room temperature until all solids weredissolved. Then, 9.5 grams AA was added in a single charge to theaqueous solution. Next, an oil solution was prepared by adding anddissolving 0.177 grams of CBr4 chain transfer agent in 180 grams of IOA.The oil solution was then added in a single charge to the reaction flaskwhile stirring to create an emulsion. Then a nitrogen purge above thereaction mixture was started at 2 liters/minute for 30 minutes. Next,the reaction flask was heated to 30° C. using controlled heat lampsfollowed by addition of 0.5 milliliters of FS solution then 0.095 gramsSMBS and 0.38 grams KPS to initiate the reaction. The temperature wasobserved to increase to about 60° C. due to the polymerization exotherm.The reaction was held at this temperature for one hour, then allowed tocool to room temperature, followed by addition of 1.32 grams of LiOHdissolved in 16.43 grams of deionized water. After filtering throughcheese cloth an emulsion polymer composition having a solids content ofabout 44% was obtained

Preparation of Composition Examples 1-2

Composition Examples 1-2, comprising emulsion polymers EP 2-EP 3,containing the amino acid L-Cys in various amounts as a CTA andcrosslinker, were prepared in a manner similar to that described forComparative Composition Example 1 with the following modifications. TheCBr4 was replaced with various amounts of L-Cys and the L-Cys was addedalong with the AA to the aqueous phase. The mol % with respect to(w/r/t) monomer was calculated by taking the ratio of amount of CTA inmoles to total amount of monomers in moles (0.05 moles of CTA per 100moles of monomers). Wt % with respect to (w/r/t) monomer is calculatedin a similar way but using weight of the CTA and monomers (0.03 g of CTAper 100 g of monomers).

TABLE 1 Composition CTA CTA amount CTA amount Composition Emulsionamount (mol % w/r/t (wt % w/r/t Example Polymer CTA (grams) monomer)monomer) CE 1 EP 1 CBr4 0.177 0.05 0.09 1 EP 2 L-Cys 0.033 0.02 0.02 2EP 3 L-Cys 0.066 0.05 0.03

Preparation of Comparative Tape Example 1 and Tape Examples 1-2

Comparative Composition Example 1 and Composition Examples 1-2 werecoated onto a primed polyester film then dried in an oven at 100° C. forfive minutes to provide tape samples having final adhesive thicknessesand properties as shown in Table 2.

TABLE 2 Composition Dry 180° CTA Adhesive Peel Thickness Adhe- Emul-amount (wt micro- Shear sion Tape sion % w/r/t meters (minutes) SS oz/inEx. Polymer CTA monomer) (inches) SS FB (N/cm) CE 1 EP 1 CBr4 0.09 27.9 548  66 19.6 (0.011)  (2.15) 1 EP 2 L-Cys 0.02 30.5 2439 1109 16.3(0.0012) (1.78) 2 EP 3 L-CYs 0.03 27.9 1747  819 15.4 (0.011)  (16.9) 

Preparation of Composition Examples 3-6

Composition Examples 3-6 were prepared by combining EP 1 and selectedamounts of L-Hist crosslinker in a glass jar which was then sealed andplaced on a roller for at least one hour.

Preparation of Comparative Composition Example 2 and CompositionExamples 7-8

Base Emulsion Polymer 1 was provided along with modifications made bythe addition of various amounts of L-Phenyl crosslinker. These wereprepared by mixing as described for Composition Examples 3-6.

Preparation of Comparative Composition Example 3 and CompositionExamples 9-10

Base Emulsion Polymer 2 was provided along with modifications made bythe addition of either L-Hist or L-Phenyl crosslinker. These wereprepared by mixing as described for Composition Examples 3-6.

Preparation of Comparative Tape Examples 2 and 3, and Examples 3-10

Comparative Composition Examples 2 and 3, and Composition Examples 3-10,were used to prepare tape samples as described for Comparative TapeExample 1 and Tape Examples 1-2 above. Tape Example 6 was prepared thesame way as Tape Example 5 with the following modification. Thecomposition was kept at room temperature for 16 days then used toprepare tape samples without any further mixing. Adhesive compositions,thicknesses, and tape properties are shown in Tables 3 and 4 below. Thewt.-% of the amino acids was calculated by taking the ratio of amount ofamino acids in weight to the total weight of aqueous emulsion polymer,including the weight of the water. The wt.-% solids can be determined byadjusting the weight to account for the removal of the aqueous phase bydrying. For example, when the aqueous pressure sensitive adhesivecontains 50 wt.-% aqueous phase, the concentration of amino acid as apercentage of solids doubles upon removal of the aqueous phase. (1 wt.-%solution=2 wt.-% solids)

TABLE 3 Composition Crosslinker Emulsion L- Emulsion CrosslinkerComposition Emulsion Polymer Phenyl L-Hist Polymer L-Phenyl L-HistExample Polymer (grams) (grams) (grams) (wt %) (wt %) (wt %) CE 1 EP 1N/A 0    0    100    0   0   3 EP 1 20.2 0    0.074 99.63 0   0.37 4 EP1 40.0 0    0.436 98.91 0   1.09 5 EP 1 59.9 0    0.668 98.88 0   1.12 6EP 1 59.9 0    0.668 98.88 0   1.12 CE 2 BP 1 N/A 0    0    100    0  0   7 BP 1 10   0.041 0    99.59 0.41 0   8 BP 1 10   0.132 0    98.681.32 0   CE 3 BP 2 N/A 0    0    100    0   0   9 BP 2 10   0.132 0   98.68 1.32 0   10  BP 2 10   0    0.124 98.76 0   1.24

TABLE 4 180° Peel Dry Adhesive Adhesion Crosslinker Thickness, Shear SSTape Emulsion L-Phenyl L-Hist micrometers SS FB oz/in Example Polymer(wt %) (wt %) (inches) (minutes) (minutes) (N/cm) CE 1 EP 1 0   0   27.9548 66 19.6 (0.0011) (2.15) 3 EP 1 0   0.37 33.0 946 19.0 (0.0013)(2.08) 4 EP 1 0   1.09 27.9 10,000+ 10,000+ 16.7 (0.0011) (1.83) 5 EP 10   1.12 27.9 10,000+ 10,000+ 14.9 (0.0011) (1.63) 6 EP 1 0   1.12 30.510,000+ 17.3 (0.0012) (1.89) CE 2 BP 1 0   0   27.9 880 22.1 (0.0011)(2.42) 7 BP 1 0.41 0   30.5 8,015 24.8 (0.0012) (2.71) 8 BP 1 1.32 0  30.5 10,000+ 27.7 (0.0012) (3.03) CE 3 BP 2 0   0   35.6 77 27.4(0.0014) (3.00) 9 BP 2 1.32 0   33.0 428 29.0 (0.0013) (3.17) 10  BP 20   1.24 33.0 2756 18.8 (0.0013) (2.06)

Preparation of Composition Examples 11-13

In a glass jar, the emulsion polymer 2 (EP-2) was combined with L-Lysineamino acid, obtained from Sigma at the quantities described in thefollowing table. The glass jar was then rolled on a jar roller for twohours to ensure thorough mixing. Solutions were coated on primed BOPP(1.6 mil) and then dried in an oven at 70° C. for 7.5 minutes togenerate adhesive-coated tape with an adhesive thickness of 1 mil.

The shear strength at room temperature to fiberboard was tested in thesame manner a previously described except that 4 passes of the rollerwas used to adhere the tape to the fiberboard. The shear strength atroom temperature to fiberboard was retested after aging at 120° F. for11 days.

Emulsion Shear Shear on FB (min) after Adhesive Lysine on FB aging (11days at Ex. (g) (g) (min) 120° F.) CE4 N/A 0 19 20 11 30.1 0.242 43 1412 30.0 0.342 157 198 13 30.1 0.435 698 304

What is claimed is:
 1. A pressure sensitive adhesive compositioncomprising: a (meth)acrylic polymer comprising polymerized units of(meth)acrylic ester monomer having a Tg less than 0° C. andethylenically unsaturated monomer comprising a pendent functional groupcrosslinked with an amine or carboxylic acid group of at least one aminoacid.
 2. The pressure sensitive adhesive composition of claim 1 whereinthe pressure sensitive adhesive has a 180° peel adhesion to stainlesssteel of at least 0.25, 0.5, 1 or 1.5 N/cm.
 3. The pressure sensitiveadhesive composition of claims 1-2 wherein the (meth)acrylic polymercomprises at least 50, 55, 60, 65, 70, 75, 80, 85, 90 or 95 wt-% ofpolymerized units of (meth)acrylic ester monomer(s) having a Tg lessthan 0° C.
 4. The pressure sensitive adhesive composition of claims 1-3wherein the (meth)acrylic polymer comprises at least 50, 55, 60, 65, 70,75, 80, 85, 90 or 95 wt-% of polymerized units of (meth)acrylic estermonomer(s) having an alkyl group with 6 to 20 carbon atoms.
 5. Thepressure sensitive adhesive composition of claims 1-4 wherein thecomposition further comprises tackifying resin.
 6. The pressuresensitive adhesive composition of claims 1-5 wherein the pressuresensitive adhesive has a Tg no greater than −10° C.
 7. The pressuresensitive adhesive composition of claims 1-6 wherein the pressuresensitive adhesive has a storage modulus of no greater than 0.3MPa at25° C. and 1 Hertz.
 8. The pressure sensitive adhesive composition ofclaims 1-7 wherein the (meth)acrylic polymer comprises 1 to 15 wt-% ofthe polymerized units of the ethylenically unsaturated monomercomprising the pendent functional group.
 9. The pressure sensitiveadhesive composition of claims 1-8 wherein the pendent functional groupis a carboxylic acid group, an amine group, an amide group, or acombination thereof.
 10. The pressure sensitive adhesive composition ofclaims 1-9 wherein the amino acid comprises a side chain having a groupselected from a hydrophobic group, a cyclic group, or a chain transferagent group.
 11. The pressure sensitive adhesive composition of claim 10wherein the chain transfer agent group is selected from SH, SeH, or OH.12. An article comprising the composition of claims 1-11 disposed on asubstrate.
 13. The article of claim 12 wherein the substrate is arelease liner or backing.
 14. A pressure sensitive adhesive compositioncomprising: an aqueous phase; a surfactant; a (meth)acrylic polymercomprising polymerized units of (meth)acrylic ester monomer having a Tgless than 0° C. and ethylenically unsaturated monomer comprising apendent functional group wherein the pendent functional group forms anionic or covalent bond with an amine or carboxylic acid group; and an atleast one amino acid crosslinker.
 15. The pressure sensitive adhesivecomposition of claim 14 wherein the pressure sensitive adhesivecomposition is further characterized by any one or combination of claims1-11.
 16. A pressure sensitive adhesive comprising the dried andcrosslinked composition of claims 14-15.
 17. A method of making apressure sensitive adhesive composition comprising: a) providing anaqueous pressure sensitive adhesive composition comprising a(meth)acrylic polymer comprising polymerized units of an ethylenicallyunsaturated monomer comprising a pendent functional group that forms anionic or covalent bond with an amine or carboxylic acid group; b)combining the aqueous pressure sensitive adhesive composition with anamino acid; c) coating the aqueous pressure sensitive adhesivecomposition prepared in (b) onto a substrate; e) drying the pressuresensitive adhesive composition and thereby crosslinking the pendentfunctional groups of the (meth)acrylic polymer with the amino acid. 18.A method of making a pressure sensitive adhesive composition comprising:a) making a monomer mixture comprising (i) low Tg (meth)acrylic monomer;b) combining the monomer mixture with an aqueous phase comprising (i)water, (ii) an amino acid comprising a side chain having a chaintransfer group selected from SH, SeH, or OH; (iii) a surfactant, and(iv) a free-radical initiator; wherein monomer having a functional groupthat is reactive with an amine or carboxylic acid group is present inthe monomer mixture, the aqueous phase, or a combination thereof; c)polymerizing the monomers; d) coating the aqueous pressure sensitiveadhesive composition onto a substrate; e) drying the pressure sensitiveadhesive composition and thereby crosslinking the pendent functionalgroups of the (meth)acrylic polymer with the amino acid.
 19. The methodof claim 18 wherein the amino acid functions as a chain transfer agentand crosslinker.
 20. The method of claims 17-19 wherein the pressuresensitive adhesive composition is further characterized by any one orcombination of claims 1-11.
 21. The pressure sensitive adhesivecomposition of claims 1-9 wherein the amino acid comprises anelectrically charged side chain.
 22. An article comprising thecomposition of claim 21 disposed on a substrate.
 23. The article ofclaim 22 wherein the substrate is a release liner or backing.
 24. Thepressure sensitive adhesive composition of claim 14 wherein the pressuresensitive adhesive composition is further characterized by claim
 21. 25.A pressure sensitive adhesive comprising the dried and crosslinkedcomposition of claim
 24. 26. The pressure sensitive adhesive of any ofthe previous claim wherein the pressure sensitive adhesive has a shearstrength after aging for 10 days at 120° F. of greater than or equal tothe initial shear strength.